1. Field of the Invention
The present invention relates to an apparatus for aiding in the transfer of a Bingham solid, e.g., an aqueous explosive composition of the water gel or water-in-oil emulsion type, or an aqueous composition useful for stimulating wells by the hydraulic fracturing method, through a long conduit, i.e., a pipe or hose, and to a method of using the apparatus.
2. Description of the Prior Art
A body of prior art exists on apparatuses which inject lubricating liquids of lower viscosity into conduits carrying liquids of higher viscosity, e.g., petroleum oils, to reduce the pressure drop therein. In this art, the liquid of lower viscosity is injected into the conduit by any of a number of devices so as to form an annular layer between the inner wall of the conduit and the liquid of higher viscosity flowing through the conduit. U.S. Pat. Nos. 2,821,205 (E. G. Chilton et al.) and 3,414,004 (Bankston) are examples of this art.
A smaller body of prior art deals with similar devices for reducing the pressure drop in conduits carrying Bingham solids in plug flow, or materials that, depending on the temperature, are viscous liquids or Bingham solids. U.S. Pat. Nos. 3,886,972 (P. R. Scott et al.), 3,826,279 (E. Verschuur), 4,273,147 (R. S. Olney), and 4,259,977 (J. W. Brockington) typify this art, the latter two patents dealing with the transportation and placement of Bingham solids that are emulsion explosives.
The term "Bingham solid" as used herein to refer to the material to be moved through the long conduit, denotes a substance of gelatinous, plastic, or semisolid consistency that will flow without delay only after having had imposed on it a shear stress that exceeds some finite critical value. Solids which have this property nevertheless may deform gradually over a period of time under stresses that are much lower than this critical stress for flow without delay.
In an application such as that in which a Bingham solid is delivered down a deep oil or gas well to extend and prop a hydraulic fracture in the producing formation and thereby stimulate production from the well, the best results are obtained with the very high delivery rates that can be achieved most easily if the pressure drop in the conduit is very low. Such Bingham solids can be, for example, gelled water containing suspended particles of hard materials (e.g., sand, sintered bauxite, or walnut shells) having sufficient strength to prevent complete closure of a hydraulic fracture if distributed in the fracture.
Where the material being delivered through the conduit is a Bingham solid, the pressure drop is most effectively reduced if the annulus of liquid injected into the conduit provides sufficient lubrication to permit the column of Bingham solid to slide through the conduit without undergoing appreciable deformation in shear, that is, if it moves in what is known as "plug flow". In applications where the Bingham solid is to be transferred downward through a descending section of conduit via such lubricated plug flow, it is particularly advantageous if the plug flow in the descending section can occur under the propelling force of gravity alone. The low drag required to obtain plug flow under gravity alone is also advantageous in conduits having no descending section.
In some applications, where the lubricant is immiscible with the Bingham solid, it is possible to use copious amounts of the lubricant and recover most of it by skimming it off on the downstream end of the conduit. In some other applications, where the lubricant is miscible with the Bingham solid, eventual dilution of the product with lubricant has no serious consequences. But when the Bingham solid is an explosive being delivered into a borehole or well, it is not feasible to skim off the lubricant and it is particularly important to use as little lubricant as possible in order to maintain the effective strength and sensitivity of the explosive. The spray ring apparatus used in the method of the aforementioned U.S. Pat. No. 4,259,977, which represents the state of the prior art in this regard, is disclosed as being used most effectively with a lubricating fluid flow rate which is from about 3% to about 5% of the flow rate of the explosive, on a weight basis. In the lubricant injectors described in the aforementioned patents, the annular passageway through which the lubricant flows has a fixed cross-section, which precludes convenient adjustment of the lubricant flow conditions within the injector itself.
Furthermore, the lubricant injectors described in the aforementioned patents do not, in general, permit a Bingham solid to slide through a descending conduit on an annulus of lubricating fluid under the influence of gravity alone, an important advantage if the Bingham solid is being delivered down a deep hole. For example, the aforementioned U.S. Pat. No. 4,273,147 (Olney) describes an experiment in which an emulsion of density 1.25 was pumped through a 25.4 mm hose at a rate of 27 liters per minute, using the lubricant injector described in that patent. The resulting pressure drop was about 15.8 kPa per meter. Since gravity could have produced a pressure drop of only 12.3 kPa per meter, the device of Olney would not have permitted his Bingham solid to slide through his hose at the stated rate under the influence of gravity alone, nor is there any reason to believe that it could have moved through the hose at all under the influence of gravity alone.
A lubricant injector whose use would permit a Bingham solid to slide through a descending conduit on an annulus of lubricating fluid under the influence of gravity alone would permit Bingham solid to be delivered through a descending conduit of very great additional length with no additional pumping pressure. In some circumstances, where the descending conduit is at the outlet of a hopper, no pumping pressure at all would be required.